TWI417906B - Functionally graded rare earth permanent magnet - Google Patents

Description

Functional graded rare earth permanent magnet

The present invention relates to a highly efficient rare earth permanent magnet having a graded function, which has a surface coercive force higher than the inside and has an effective improved thermal resistance.

Due to the excellent magnetic properties, the application range of Nd-Fe-B permanent magnets has been found to increase. In order to meet today's environmental problems, the use of magnets has expanded to cover household appliances, industrial installations, electric vehicles and wind turbines. This requires further improvement in the performance of the Nd-Fe-B magnet.

When the temperature rises, the coercive force of the Nd-Fe-B magnet decreases. Therefore, the service temperature of the magnet is affected by the magnitude of the coercive force and the magnetic conductance of the magnetic circuit. The magnet must have a high coercive force for use at high temperatures. A number of methods have been suggested for increasing the coercive force, including grain refinement, the use of alloy compositions with increased amounts of Nd, and the addition of effective elements. The most common method today is to partially replace the alloy composition of Nd with Dy or Tb. By replacing some of the Nd in the Nd ₂ Fe _{1 4} B compound with Dy or Tb, the anisotropic magnetic field and coercive force of the compound increase. On the other hand, substitution of Dy or Tb reduces the saturation magnetic polarization of the compound. Therefore, as long as the coercive force is reduced by this method, the reduction of residual magnetism cannot be avoided.

JP 3,471,876 discloses a rare earth magnet having improved corrosion resistance, comprising at least one rare earth element R, and which is obtained by fluorination treatment in a fluorine gas atmosphere or a fluorine gas-containing atmosphere to be on the surface of the magnet An RF ₃ compound or a RO _x F _y compound (wherein the values of x and y satisfy 0 < x < 1.5 and 2 x + y = 3) or a mixture having R in the constituent phase, and heat treatment at 200 to 1,200 ° C is formed.

JP-A 2003-282312 discloses R-Fe-(B,c) sintered magnets (wherein R is a rare earth element, at least 50% of which is Nd and/or Pr), which has improved magnetizability, and It is prepared by mixing an alloy powder of R-Fe-(B, C) sintered magnets with a rare earth fluoride powder such that the powder mixture contains 3 to 20% by weight of rare earth fluoride (the rare earth group is preferably Dy and / or Tb), the powder mixture is aligned in the magnetic field, and compacted and sintered, the main phase is mainly composed of Nd ₂ Fe _{1 4} B grains, and the grain boundary phase is at the main phase grain boundary or grain boundary 3 Formed at the phase, the grain boundary phase contains a rare earth fluoride, and the rare earth fluoride accounts for 3 to 20% by weight of the total sintered magnet. Specifically, R-Fe-(B, C) sintered magnets are provided (where R is a rare earth element, at least 50% of which is Nd and/or Pr), wherein the magnet comprises mainly Nd ₂ Fe _{1 4} B crystal The main phase of the grain composition, and the grain boundary phase comprises a rare earth fluoride, the main phase comprising Dy and/or Tb, the main phase comprising a region, wherein the concentration of Dy and/or Tb is lower than Dy and/or in the main main phase The average concentration of Tb.

However, these suggestions are not sufficient for improved coercive force.

JP-A 2005-11973 discloses a rare earth-iron-boron type magnet which is obtained by placing a magnet in a vacuum chamber, an element M or an alloy containing an element M (M represents one or more selected from Pr, Deposition of Dy, Tb and Ho rare earth elements), by physically evaporating or atomizing all or part of the magnet surface in a vacuum chamber, and performing pack cementation, element M diffuses and from the surface of the magnet Permeating into the interior of the magnet to at least one depth corresponding to the radius of the grains exposed on the outermost surface of the magnet to form a grain boundary layer rich in element M. The concentration of the element M on the grain boundary layer is higher at the position closest to the surface of the magnet. Therefore, the magnet has a grain boundary layer rich in the element M by the element M diffused from the surface of the magnet. The coercive force Hcj and the element M content in the total magnet have the following relationship: Where Hcj is the coercive force, the unit is MA/m, M is the content of the element M in the total magnet (% by weight), and 0.05 M 10. However, the productivity of this method is very low and very impractical.

SUMMARY OF THE INVENTION An object of the present invention is to provide a rare earth permanent magnet having a graded function, which has a surface coercive force higher than the inside, and whose thermal resistance is effectively improved.

In general, the magnet built into the magnetic circuit does not exhibit the same magnetic permeability in the entire magnet, that is, the inside of the magnet has a distribution of the diamagnetic field. For example, if a plate magnet has a magnetic pole on a wide surface, the center of its surface receives the largest diamagnetic field. In addition, the magnet surface receives a large diamagnetic field compared to the inside. Therefore, when the magnet is exposed to high temperatures, demagnetization occurs from its surface layer. Regarding R-Fe-B sintered magnets (wherein R is one or more selected from the group consisting of rare earth elements containing Sc and Y), typically Nd-Fe-B sintered magnets, the inventors have discovered that when Dy and/or Tb and fluorine Absorbed and infiltrated into the magnet from the surface of the magnet, Dy and / or Tb and fluorine are only enriched at the interface adjacent to the grain, so that the magnet has a stepped function of surface coercive force higher than the internal, and especially the magnet The coercive force increases from the center to the surface. Therefore, the thermal impedance is effectively improved.

Accordingly, the present invention provides a functional graded rare earth permanent magnet in the form of a sintered magnet having an alloy composition R ¹ _a R ² _b T _c A _d F _e O _f M _g , wherein R ¹ is selected from the group consisting of containing Sc and Y, but not including at least one of the rare earth elements of Tb and Dy, R ² is one or both of Tb and Dy, T is one or both of iron and cobalt, and A is boron and carbon. In one or both, F is fluorine, O is oxygen, and M is selected from the group consisting of Al, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, At least one of Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W, a to g represents the atomic percentage of the corresponding element in the alloy and the range is as follows: 10 a+b 15,3 d 15,0.01 e 4, 0.04 f 4,0.01 g 11, the remainder is c, the magnet has a center and a surface. The grain boundaries surround the main phase grains of the (R ¹ , R ² ) ₂ T _{1 4} A tetragonal system in the sintered magnet. The concentration of R contained in the grain boundaries ^{2 / (R 1 + R 2} ) is higher than the average concentration of R contained in the main phase crystal grains of ^{2 / (R 1 + R 2} ) a. The distribution of R ² causes its concentration to increase evenly from the center of the magnet to the surface. The (R ¹ , R ² ) oxyfluoride has grain boundaries extending from the surface of the magnet to a grain boundary region having a depth of at least 20 μm. The coercive force of the surface of the magnet is higher than the inside of the magnet.

In a preferred embodiment, the oxyfluoride at the grain boundary (R ¹ , R ² ) comprises Nd and/or Pr, and the Nd and/or Pr pair (R ¹ + R ² ) contained in the oxyfluoride at the grain boundary. The atomic ratio is higher than the atomic ratio of Nd and/or Pr pairs (R ¹ + R ² ) contained at the grain boundaries but excluding R ³ oxyfluorides and oxides, wherein R ³ is selected from the group consisting of rare earths containing Sc and Y At least one element of a family element.

In a preferred embodiment, R ¹ comprises at least 10 atomic percent of Nd and/or Pr; T comprises at least 60 atomic percent iron; and A comprises at least 80 atomic percent boron.

The permanent magnet of the present invention has a magnetic structure in which the surface layer has a higher coercive force than the inside and has an effectively improved thermal impedance.

The rare earth permanent magnet of the present invention is in the form of a sintered magnet having the alloy composition of the formula (1).

R ¹ _a R ² _b T _c A _d F _e O _f M _g (1) wherein R ¹ is at least one element selected from the group consisting of Sc and Y but not including Tb and Dy, and R ² is Tb and One or both of Dy, T is one or both of iron and cobalt, A is one or both of boron and carbon, F is fluorine, O is oxygen, and M is selected from Al, At least any of Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W An element, a to g, represents the atomic percentage of the corresponding element in the alloy and ranges as follows: 10 a+b 15,3 d 15,0.01 e 4, 0.04 f 4,0.01 g 11, the remaining is c.

Specifically, R ¹ is selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, _Er , Yb, and Lu. Conveniently, R ¹ comprises Nd and/or Pr as the main component, and the amount of Nd and/or Pr in R ¹ is preferably at least 10 atom%, more preferably at least 50 atom%. R ² is one or both of Tb and Dy.

The total amount (a+b) of R ¹ and R ² is 10 to 15 at%, as described above, and preferably 12 to 15 at%. The amount (b) of R ² is from 0.01 to 8 atom%, preferably from 0.05 to 6 atom%, and more preferably from 0.1 to 5 atom%.

The amount of T (c), wherein T is Fe and/or Co, preferably at least 60 atom%, and more preferably at least 70 atom%. Although cobalt (i.e., 0 atom%) may be omitted, the amount of cobalt may be included in an amount of at least 1 atom%, preferably at least 3 atom%, more preferably at least 5 atom%, in order to improve temperature stability of residual magnetism or other purposes.

A is boron and/or carbon, and preferably contains at least 80 atom% (more preferably at least 85 atom%) of boron. The amount (d) of A is from 3 to 15 at%, as described above, preferably from 4 to 12 at%, and more preferably from 5 to 8 at%.

The amount (e) of fluorine is from 0.01 to 4 atom%, as described above, preferably from 0.02 to 3.5 atom%, and more preferably from 0.05 to 3.5 atom%. When the amount of fluorine is too low, the coercive force does not increase. Too high a fluorine content will change the grain boundary phase and cause a decrease in coercive force.

The oxygen amount (f) is from 0.04 to 4 atom%, as described above, preferably from 0.04 to 3.5 atom%, and more preferably from 0.04 to 3 atom%.

The amount (g) of the metal element M is from 0.01 to 11 atom%, as described above, preferably from 0.01 to 8 atom%, and more preferably from 0.02 to 5 atom%. The other metal element M is present in an amount of at least 0.05 atomic %, and especially at least 0.1 atomic %.

The sintered magnet has a center and a surface. In the present invention, the distribution of the constituent elements F and R ² in the sintered magnet is such that the concentration thereof increases evenly from the center to the surface of the magnet. Specifically, the concentrations of F and R ² are highest at the surface of the magnet and slowly toward the center of the magnet. There may be no fluorine present in the center of the magnet, as the present invention only requires the presence of fluorofluorides of R ¹ and R ² at the grain boundaries in the grain boundary region from the surface of the magnet to a depth of at least 20 μm, typically (R ¹ _{1 - x} R ² _x )OF (where x is the number from 0 to 1). The grain boundary is surrounded by the main phase grains of the (R ¹ , R ² ) ₂ T _{1 4} A tetragonal system in the sintered magnet, and the R ² /(R ¹ +R ² ) concentration contained in the grain boundary is on average higher than The concentration of R ² /(R ¹ +R ² ) contained in the main phase grains.

In a preferred embodiment, the oxyfluoride of (R ¹ , R ² ) present at the grain boundary comprises Nd and/or Pr, and Nd and/or Pr pairs in the oxyfluoride contained in the grain boundaries ( The atomic ratio of R ¹ + R ² ) is higher than the atomic ratio of Nd and/or Pr pair (R ¹ + R ² ) contained in the grain boundary but R ³ oxyfluoride and oxide, wherein R ³ is selected from the group consisting of At least one of the rare earth elements of Sc and Y.

The rare earth permanent magnet of the present invention is produced by absorbing and infiltrating Tb and/or Dy and fluorine from the surface of the R-Fe-B sintered magnet. The R-Fe-B sintered magnet can then be fabricated by conventional methods, including crushing, grinding, compacting, and sintering the mother alloy.

The master alloy used herein contains R, T, A and M. R is at least one element selected from the group consisting of rare earth elements of Sc and Y. R is typically selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu. Conveniently, R contains Nd, Pr and Dy as main components, and these rare earth elements containing Sc and Y are preferably present in an amount of 10 to 15 atom%, more preferably 12 to 15 atom%, of the total alloy. More conveniently, R comprises one or both of Nd and Pr in an amount of at least 10 atom%, in particular at least 50 atom%, of all R. T is one or both of Fe and Co, and the amount of Fe contained is preferably at least 50 atom%, and more preferably at least 65 atom%, of the total alloy. A is one or both of boron and carbon, and the amount of boron contained is preferably 2 to 15 atom%, and more preferably 3 to 8 atom%, based on the total alloy. M is selected from the group consisting of Al, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, At least one element of Ta and W. The amount of M contained may be from 0.01 to 11 atom%, and preferably from 0.1 to 5 atom%, based on the total alloy. The remainder consists of concomitant impurities such as N and O.

The master alloy is prepared by melting a metal or alloy supply in a vacuum or an inert gas atmosphere (typically an argon atmosphere), casting the melt to a flat mold or a book mold or performing sheet casting. Another method that can be used is a two-alloy method comprising separately forming an alloy of a composition of a compound of R ₂ Fe _{1 4} B constituting a main phase of a related alloy and an alloy of R rich in a liquid phase at a sintering temperature. , crush it, then weigh and mix. An alloy of approximately the composition of the main phase, if necessary, is homogenized to increase the amount of the R ₂ Fe _{1 4} B compound phase, since α-Fe may remain depending on the cooling rate and alloy composition during casting. The homogenization treatment is carried out at a temperature of 700 to 1,200 ° C for at least 1 hour in a vacuum or in an Ar atmosphere. The R-rich alloy as a liquid phase adjuvant can be applied by so-called melt quenching or flaky casting techniques, as well as the casting techniques described above.

The master alloy is usually crushed to a size of 0.05 to 3 mm, preferably 0.05 to 1.5 mm. The crushing step uses a Brown mill or hydrogenation pulverization, and when these alloys are sheet castings, hydrogenation pulverization is preferably used. The coarse powder is then finely divided into a size of 0.2 to 30 μm, preferably 0.5 to 20 μm, for example, by a jet mill using nitrogen under pressure. The amount of oxygen in the sintered body can be controlled by mixing a small amount of oxygen with pressurized nitrogen at this time. The amount of oxygen of the final sintered body, which is oxygen which is added during the preparation of the ingot and oxygen which is changed from the fine powder to the sintered body, is preferably from 0.04 to 4 atom%, more preferably from 0.04 to 3.5 atom%.

The powder is then compacted in a compression casting machine under a magnetic field and placed in a sintering furnace. Sintering is carried out in a vacuum or an inert gas atmosphere at a temperature of 900 to 1,250 ° C, preferably 1,000 to 1,100 ° C. The obtained sintered magnet contains 60 to 99% by volume, preferably 80 to 98% by volume, of the tetragonal R ₂ Fe _{1 4} B compound as a main phase, and the balance is 0.5 to 20% by volume of the R-rich phase, 0 to 10% by volume of the B-rich phase, 0.1 to 10% by volume of the R oxide, and at least one of the carbides, nitrides and hydroxides accompanying the impurities or a mixture or composite thereof.

After the rare earth elements (typically Tb and/or Dy) and the fluorine atoms absorb and penetrate into the magnet to impart a magnetic structure with a surface coercive force higher than the center, the sintered block is machine-cut into a magnet of a given shape.

For a typical treatment, a powder comprising Tb and/or Dy and a fluorine atom is disposed on the surface of the magnet. The powder-filled magnet is in a vacuum or an inert gas (such as Ar or He) at a temperature not higher than the sintering temperature (referred to as Ts), preferably 200 ° C to (Ts - 5) ° C, especially 250 ° C to (Ts - 10) At ° C, heat treatment is carried out for about 0.5 to 100 hours, preferably about 1 to 50 hours. By heat treatment, Tb and/or Dy and fluorine atoms permeate from the surface of the magnet, and the rare earth oxide in the sintered magnet reacts with fluorine to form an oxyfluoride.

The oxyfluoride of R (containing the rare elements of Sc and Y) in the magnet is typically ROF, although it generally represents an oxyfluoride comprising R, oxygen and fluorine up to the cost of the invention, which comprises RO _m F _n ( Where m and n are positive numbers) and a modified or stabilized form of RO _m F _n wherein a portion of R is replaced by a metal element.

The amount of fluorine absorbed in the magnet at this time varies depending on the composition of the powder to be used and the particle size, the proportion of the powder surrounding the surface of the magnet during heating, the time and temperature of heating, but the amount of fluorine absorbed is preferably 0.01 to 4 atom%, more preferably 0.05 to 3.5 atom%. The amount of fluorine absorbed is preferably from 0.1 to 3.5 atom%, particularly from 0.15 to 3.5 atom%, from the viewpoint of increasing the coercive force of the surface layer. For absorption, the amount of fluorine supplied to the surface of the magnet is preferably from 0.03 to 30 mg/cm ² surface, more preferably from 0.15 to 15 mg/cm ² surface.

By heat treatment, the Tb and/or Dy components are also concentrated at adjacent grain boundaries to enhance anisotropy. The total amount of the Tb and Dy components absorbed in the magnet is preferably from 0.005 to 2 atom%, more preferably from 0.01 to 2 atom%, still more preferably from 0.02 to 1.5 atom%. For absorption, the total amount of the Tb and Dy components supplied to the surface of the magnet is preferably from 0.07 to 70 mg/cm ² surface, more preferably from 0.35 to 35 mg/cm ² surface.

Therefore, the coercive force of the surface layer of the produced magnet is higher than that of the inside of the magnet. Although there is no special requirement for the difference in coercive force between the surface layer and the inside, the fact that the difference between the surface layer and the internal magnetic permeability is about 0.5 to 30% suggests that the surface coercive force of the magnet is higher than the internal (at least 2 mm depth from the surface of the magnet). 5 to 150%, preferably 10 to 150%, more preferably 20 to 150%.

The coercive force of the different places in the magnet is measured as follows: the magnet is cut into small portions and the magnetic properties of the small portions are measured.

The permanent magnet material of the present invention has a graded function in which the surface layer coercive force is higher than the inside, and can be used as a permanent magnet having improved thermal resistance (especially in applications including an electric motor and a reading actuator).

Instance

The examples of the invention are described below for illustration and not for limitation.

Example 1 and Comparative Example 1

An alloy in the form of a thin plate is produced by using Nd, Cu, Al and Fe metals and ferroboron having a purity of at least 99% by weight, each weighing a certain amount, and melting it at a high frequency in an Ar atmosphere. And a single chill roll (sheet casting technique) that casts the melt into copper. The alloy is composed of 13.5 atom% Nd, 0.5 atom% Al, 0.4 atom% Cu, 6.0 atom% B, and the balance Fe.

The alloy was ground to a size of 30 mesh by hydrogenation techniques. The coarse powder was finely divided by a jet mill using a nitrogen gas under pressure to have a mass-based median diameter of 3.7 μm. Under air shielding, the fine powder was aligned in a nitrogen atmosphere at a 15 kOe magnetic field and pressed at a pressure of about 1 ton/cm ² . The compact was then placed in a sintering furnace using an Ar atmosphere and sintered at 1,050 ° C for 2 hours to obtain a magnetic block. A disk having a diameter of 20 mm and a thickness (orientation direction) of 14 mm was machine-cut on all surfaces of the magnet block. This magnet has an average permeability of 2. The magnet is then washed with an alkaline solution, deionized water, aqueous acetic acid and deionized water, and dried.

Next, the cesium fluoride powder having an average particle diameter of 5 μm was dispersed in ethanol and mixed at 50% by weight. The magnet was immersed in the dispersion for 1 minute while the dispersion was sonicated at 48 kHz, absorbed and immediately dried with hot air. The supply of cesium fluoride was 0.8 mg/cm ² . Thereafter, the filled magnet was subjected to an absorption treatment at 900 ° C for 1 hour in an Ar atmosphere, and then subjected to an aging treatment at 520 ° C for 1 hour and quenched to obtain a magnet within the scope of the present invention. This magnet is called M1. For comparison purposes, a magnet that was not treated with a lanthanum fluoride package but was heat treated was referred to as P1.

The magnetic properties (residual magnet Br, coercive force Hcj) of the magnets M1 and P1 were measured, and the results are shown in Table 1. The composition of the magnets is shown in Table 2. The magnet M1 of the present invention exhibits a magnetic property substantially comparable to that of the magnet P1 which is subjected to heat treatment without being subjected to a barium fluoride coating. These magnets were treated at different temperatures ranging from 50 to 200 ° C for 1 hour, after which the total magnetic flux was measured. The temperature at the total magnetic flux that is 5% lower than the total magnetic flux at room temperature is the maximum service temperature. The results are also shown in Table 1. Although the magnets M1 and P1 have substantially the same coercive force, the maximum operating temperature of the magnet M1 is 20 ° C higher than that of the magnet P1.

The magnets M1 and P1 are cut in the alignment direction (14 mm thickness direction) to a thickness of 0.5 mm and cut into 4 x 4 mm. The coercive force of the small magnet portion of 4 mm x 4 mm x 0.5 mm (thickness) was measured, and the resulting coercive force is shown in Fig. 1 with respect to the distance from the original magnet surface. The magnet P1 maintains a constant magnetic force, while the magnet M1 has the highest magnetic force at the surface layer and drops to the lowest inside. The lowest coercive force inside the magnet M1 is the same as that of the P1 system. Based on these small magnet portions showing the coercive force from the surface layer of the magnet to the inside, it was confirmed that the magnet M1 of the present invention has a coercive force distribution in which the coercive force is highest on the surface layer.

The surface layers of the magnets M1 and P1 were analyzed by electronic microanalysis (EPMA), and the Dy distribution images are shown in Figs. 2a and 2b. Since the source of the alloy used to make the magnet does not have Dy, the bright contrast point indicates that Dy was not found in the P1 image. In contrast, the magnet M1 performs a treatment for absorbing cesium fluoride, which is also rich in Dy only at the grain boundaries. The concentration of Dy and F in the magnet M1 subjected to the Dy infiltration treatment is shown in Fig. 3 with respect to the depth from the surface. It can be seen from this that Dy and F are rich in grain boundaries and are lowered toward the inside of the magnet.

Fig. 4 shows a distribution image of Nd, O and F, which is taken in the same manner as Fig. 2. It can be understood that once fluorine is absorbed, it reacts with cerium oxide present in the magnet to form yttrium oxyfluoride.

These data confirm that the magnet is characterized by a Dy-rich grain boundary, oxyfluoride dispersion, Dy and F graded concentrations, and the internal coercive force distribution of the magnet shows that the addition of a small amount of Dy provides good thermal impedance.

Example 2 and Comparative Example 2

An alloy in the form of a thin plate is produced by using Nd, Dy, Cu, Al and Fe metals and ferroboron having a purity of at least 99% by weight, each weighing a certain amount, and high frequency in an Ar atmosphere Melting, and a single chill roll (sheet casting technique) that casts the melt into copper. The alloy is composed of 12.0 at% Nd, 1.5 at% Dy, 0.5 at% Al, 0.4 at% Cu, 6.0 at% B, and the balance Fe.

The alloy was ground to a size of 30 mesh by hydrogenation techniques. The coarse powder was finely divided by a jet mill using a nitrogen gas under pressure to have a mass-based median diameter of 4.2 μm. Under air shielding, the fine powder was aligned in a nitrogen atmosphere at a 15 kOe magnetic field and pressed at a pressure of about 1 ton/cm ² . The compact was then placed in a sintering furnace using an Ar atmosphere and sintered at 1,060 ° C for 2 hours to obtain a magnetic block. A disk having a diameter of 10 mm and a thickness (orientation direction) of 7 mm was machine-cut on all surfaces of the magnet block. This magnet has an average permeability of 2. The magnet is then washed with an alkaline solution, deionized water, aqueous nitric acid and deionized water, and dried.

Next, the cesium fluoride powder having an average particle diameter of 10 μm was dispersed in deionized water and mixed at 50% by weight. The magnet was immersed in the dispersion for 1 minute while the dispersion was sonicated at 48 kHz, absorbed and immediately dried with hot air. The supply of cesium fluoride is 1.2 mg/cm ² . Thereafter, the filled magnet was subjected to an absorption treatment at 800 ° C for 5 hours in an Ar atmosphere, and then subjected to an aging treatment at 510 ° C for 1 hour and quenched to obtain a magnet within the scope of the present invention. This magnet is called M2. For comparison purposes, a magnet that was treated without a barium fluoride coating but was heat treated was referred to as P2.

The magnetic properties (Br, Hcj) of the magnets M2 and P2 and the maximum operating temperature defined in Example 1 were measured. The results are shown in Table 1. The composition of the magnets is shown in Table 2. Compared to the magnet P2, the magnet M2 of the present invention exhibits substantially equal residual magnetism, high coercive force, and a maximum operating temperature rise of 45 °C. The distribution of Tb and F in magnets M2 and P2 was analyzed by EPMA. The distributions of Tb and F in magnets M2 and P2 were the same as those of Dy and F in Example 1. The distribution of the coercive force of a small portion cut out from the magnet was measured in the same manner as in Example 1.

These data confirm that the magnet is characterized by a rich concentration of Tb at the grain boundaries, dispersion of oxyfluoride, fractional concentrations of Tb and F, and the distribution of the coercive force inside the magnet shows that the addition of a small amount of Tb provides good thermal impedance.

Examples 3-7 and Comparative Examples 3-7

An alloy in the form of a thin plate is produced by using Nd, Pr, Dy, Al, Fe, Cu, Co, Ni, MO, Zr and Ti metals and ferroboron having a purity of at least 99% by weight, each weighing A given amount, a high-temperature melting in an Ar atmosphere, and a single cooling roll (sheet-like casting technique) in which the melt was cast into copper. The alloy is composed of 11.5 atom% Nd, 1.0 atom% Pr, 1.0 atom% Dy, 0.5 atom% Al, 0.3 atom% Cu, 1.0 atom% M' (=Cr, Ni, Mo, Zr or Ti), 5.8 atom% B And the remainder is composed of Fe.

The alloy was ground to a size of 30 mesh by hydrogenation techniques. The coarse powder was finely divided by a jet mill using a nitrogen gas under pressure to have a mass-based median diameter of 5.1 μm. The fine powder was aligned in a nitrogen atmosphere at a 15 kOe magnetic field and compacted at a pressure of about 1 ton/cm ² . The compact was then placed in a sintering furnace using an Ar atmosphere and sintered at 1,060 ° C for 2 hours to obtain a magnetic block. A disk having a diameter of 10 mm and a thickness (orientation direction) of 7 mm was machine-cut on all surfaces of the magnet block. This magnet has an average permeability of 2. The magnet is then washed with an alkaline solution, deionized water, aqueous nitric acid and deionized water, and dried.

Subsequently, the magnet was immersed in a 50 wt% ethanol dispersion of a 90:10 cesium fluoride/yttria powder mixture for 11 minutes while sonicating the dispersion at 48 kHz. The lanthanum fluoride and cerium oxide powders each have an average particle diameter of 10 μm and 1 μm. The magnet was absorbed and dried in a vacuum dryer at room temperature for 30 minutes while vacuuming with a rotary pump. The amount of cesium fluoride supplied is 1.5 to 2.3 mg/cm ² . Thereafter, the filled magnet was subjected to an absorption treatment at 900 ° C for 3 hours in an Ar atmosphere, and then subjected to an aging treatment at 500 ° C for 1 hour and quenched to obtain a magnet within the scope of the present invention. These magnets are M3 to M7, where M' is Cr, Ni, Mo, Zr and Ti, respectively. For comparison purposes, magnets prepared without heat treatment of the powder set were P3 to P7.

The magnetic properties (Br, Hcj) of the magnets M3 to M7 and P3 to P7 and the maximum operating temperature defined in Example 1 were measured, and the results are shown in Table 1. The composition of the magnets is shown in Table 2. The magnets M3 to M7 of the present invention exhibit substantially equal magnetic properties as compared with the comparative magnets, and the maximum operating temperature rises by 20-30 °C. The distribution of Tb and F in the magnets M3 to M7 and P3 to P7 was analyzed by EPMA, which was the same as the distribution of Dy and F in Example 1. The distribution of the coercive force of a small portion cut out from the magnet was measured in the same manner as in Example 1.

These data prove that the electromagnets are characterized by a rich concentration of Tb at the grain boundaries, dispersion of oxyfluoride, and fractional concentrations of Tb and F, and the distribution of the coercive force inside the magnet shows that the addition of a small amount of Tb provides good thermal impedance.

The rare earth elements were measured in the following manner: the samples (produced in the examples and comparative examples) were all dissolved in aqua regia, measured by inductively coupled plasma (ICP), and measured by inert gas melting/infrared absorption spectroscopy. Analytical value of oxygen, and analysis of fluorine by steam distillation / metal colorimetric measurement.

Figure 1 is a graph showing the coercive force of the magnet M1 produced in Example 1 and the heat-treated and machine-cut magnet P1 (the magnet P1 is used for comparison) at a different point from the surface of the magnet.

Figures 2a and 2b show photomicrographs of Dy distribution images in magnets M1 and P1, respectively.

Figure 3 is a graph showing the average concentration of Dy and F in magnets M1 and P1 with respect to the depth from the surface of the magnet.

4a, 4b, and 4c show photomicrographs of the composition distribution images of Nd, O, and F in the magnet M1, respectively.

Claims (5)

A functional graded rare earth permanent magnet in the form of a sintered magnet having an alloy composition R ¹ _a R ² _b T _c A _d F _e O _f M _g , wherein R ¹ is selected from the group consisting of containing Sc and Y but not including Tb And at least one of the rare earth elements of Dy, R ² is one or both of Tb and Dy, T is one or both of iron and cobalt, and A is one or two of boron and carbon F is fluorine, O is oxygen, and M is selected from the group consisting of Al, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd. At least one of Ag, Cd, Sn, Sb, Hf, Ta, and W, a to g represents the atomic percentage of the corresponding element in the alloy and the range is as follows: 10 a+b 15,3 d 15,0.01 e 4, 0.04 f 4,0.01 g 11, the remainder is c, the magnet has a center and a surface, wherein the grain boundary surrounds the main phase grain of the (R ¹ , R ² ) ₂ T ₁ 4A tetragonal system in the sintered magnet, and is included in the grain boundary R ² / The concentration of (R ¹ + R ² ) is on average higher than the concentration of R ² /(R ¹ +R ² ) contained in the main phase grains, and the distribution of R ² causes the concentration to increase evenly from the center to the surface of the magnet (R ¹ , R ² ) oxyfluoride is present in the grain boundary region extending from the surface of the magnet to a grain boundary region having a depth of at least 20 μm, and the coercive force of the surface layer of the magnet is higher than that inside the magnet. The rare earth permanent magnet of claim 1, wherein the oxyfluoride at the grain boundary (R ¹ , R ² ) comprises Nd and/or Pr, and Nd contained in the oxyfluoride contained in the grain boundary / or the atomic ratio of Pr to (R ¹ + R ² ) is higher than the atomic ratio of Nd and / or Pr pair (R ¹ + R ² ) except for the oxyfluoride and oxide contained in the grain boundary but R ³ Wherein R ³ is at least one element selected from the group consisting of rare earth elements containing Sc and Y. A rare earth permanent magnet according to claim 1, wherein R ¹ comprises at least 10 atom% of Nd and/or Pr. A rare earth permanent magnet as claimed in claim 1 wherein T comprises at least 60 at% iron. A rare earth permanent magnet as claimed in claim 1 wherein A comprises at least 80 atomic percent boron.